首先看一下典型的NSString與CFStringRef的相互轉(zhuǎn)換

// CFStringRef to NSString *
NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;

// NSString * to CFStringRef
CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;

上面出現(xiàn)了一個關(guān)鍵字 __bridge ，這個關(guān)鍵字便是整個轉(zhuǎn)換的關(guān)鍵。Apple官方對于 __bridge 的解釋如下：

**__bridge** transfers a pointer between Objective-C and Core Foundation with no transfer of ownership.

**__bridge_retained** or **CFBridgingRetain** casts an Objective-C pointer to a Core Foundation pointer and also transfers ownership to you. You are responsible for calling CFRelease or a related function to relinquish ownership of the object.

**__bridge_transfer** or **CFBridgingRelease** moves a non-Objective-C pointer to Objective-C and also transfers ownership to ARC. ARC is responsible for relinquishing ownership of the object.

上面這些話總結(jié)起來就是：

__bridge 用于Objective-C和Core Foundation指針之間的轉(zhuǎn)換，這種轉(zhuǎn)換不會更換對象的所有權(quán)（ownership）。
__bridge_retained 或 CFBridgeRetain 用于從Objective-C到Core Foundation的指針轉(zhuǎn)換，并且會將對象的所有權(quán)（ownership）轉(zhuǎn)移，所以你需要在不再使用該對象的時候調(diào)用CFRelease方法來解除引用。
__bridge_transfer 或 CFBridgeRelease 用于將非Objective-C指針轉(zhuǎn)換為Objective-C指針，對象的所有權(quán)（ownership）會交給ARC，這時你無須擔(dān)心對象何時釋放，交給ARC去做就好了。
為什么在使用__bridge_retained進(jìn)行轉(zhuǎn)換時需要自己調(diào)用CFRelease來釋放對象，其實原因很簡單：Core Foundation的對象在ARC的管轄范圍之內(nèi)。

下面是示例代碼：

// Don't transfer ownership. You won't have to call `CFRelease`
CFStringRef str =(__bridge CFStringRef)string;
// Transfer ownership (i.e. get ARC out of the way). The object is now yours and you must call `CFRelease` when you're done with it
CFStringRef str =(__bridge_retained CFStringRef)string; // you will have to call `CFRelease`

// Don't transfer ownership. ARC stays out of the way, and you must call `CFRelease` on `str` if appropriate (depending on how the `CFString` was created)
NSString*string =(__bridge NSString*)str;
// Transfer ownership to ARC. ARC kicks in and it's now in charge of releasing the string object. You won't have to explicitly call `CFRelease` on `str`
NSString*string =(__bridge_transfer NSString*)str;

原文： http://www.outflush.com/nsstring-pointer-and-cfstringref-conversion-in-arc/

參考：http://blog.csdn.net/fightingbull/article/details/8098133